The invention pertains to the field of devices for providing gas-tight seals between rotating and stationary members. More particularly, the invention pertains to seals which are capable of providing gas-tight junctions in situations of high temperature and both rotational and longitudinal movement.
Rotating devices such as rotary kilns or calcining furnaces or the like present special problems to a designer. These problems are especially acute in rotary kilns used to convert waste matter such as garbage or sawdust into usable gas. Such devices comprise large rotating drums which are heated, either internally or externally. Raw materials are introduced into one end of the rotating drum from a stationary hopper. In the drum, the materials are carbonized and gases are driven off. At the opposite end of the drum, the gases are removed by a partial vacuum, or low pressure which results from the reaction, and any waste ash is deposited in a sealed hopper for later removal. A major problem in the process of gas production using a kiln of this design is that of providing a gas-tight seal between the huge rotating drum and the stationary assemblies at either end. Any leaks around the ends of the rotating drum, either of gas out or of air in, could be very hazardous, resulting in an explosion from air leaking into the reactor or a fire from gas leaking out.
There are several problems encountered in trying to provide such a gas-tight seal. The kiln operates at extremely high temperatures, requiring a seal which can withstand temperatures in excess of 1000.degree. F. without breaking down. The tubes at each end of the kiln are quite large, to allow the waste and gasses to pass through, and rotate around the longitudinal axis of the machine. When the kiln is heated it expands, substantially increasing its length along its logitudinal axis. The actual expansion, of course, depends upon many factors including the length and material of the reactor and the temperature at which it is operated, but as much as six or more inches could be expected.
Thus, it is an object of the invention to provide a gas-tight seal for high-temperature rotating devices which can withstand very high heat.
It is a further object of the invention to provide a gas-tight seal for such devices which can be used with large rotating members.
It is a still further object of the invention to provide a gas-tight seal which can accommodate significant longitudinal expansion of the rotating member with no deterioration in the quantity of the seal.
One type of seal which has long be used is known as the "labyrinth seal". The labyrinth seal, as its name implies, uses a labyrinth or maze of interlocking members to block the passage of the material sealed. One set of seal members is rigidly attached to the rotating part, and the other set is rigidly attached to the stationary member. The spaces between the two sets form the maze. Kolb, U.S. Pat. No. 837,850, shows an early example of such a seal. The gaps between the interfitting members may be filled with grease or oil, as in Kolb, or water (see Dow, U.S. Pat. Nos. 957,314 and 961,216). Sometimes no packing is provided, and the resistance created by the labyrinth is considered sufficient to hold the fluid in the device (see Ljungstrom, U.S. Pat. No. 1,009,425), or the members of the seal are placed nearly in actual contact to contain the fluid (Schlegel, U.S. Pat. No. 1,895,348). At least one inventor (Hults, U.S. Pat. No. 1,508,555) used a particulate solid packing, such as soda ash or bicarbonate.
Other patents of which I am aware, and which might be considered relevant, are Muller, U.S. Pat. No. 2,841,385; Lizogub et al, U.S. Pat. No. 4,290,610; and Harig, et al, U.S. Pat. No. 4,087,334. Muller is a conventional labyrinth, using two elements on the rotary kiln with a single element attached to the stationary part in between. Lizogub adds a passage for withdrawal of fluid from the seal. Harig uses a hydraulic arrangement to force the seal together to accomodate expansion and contraction of the drum.
Obviously, water or oil would not be appropriate packing for a high temperature application. Where any accomodation for longitudinal expansion is made at all, the prior-art seals (except Harig, noted above) simply widen the space between the rotating and stationary members of the labyrinth. This lessens the gas-tightness of the seal, and (if the seal is packed at all) increases the leakage of packing material.